1. Field of the Invention
The present invention relates to a method and apparatus for representing characters by means of a printing unit on a recording medium, and in particular to such a method and apparatus which enables printing of symbols corresponding to input characters, the characters being represented by a single symbol, a series of symbols in sequence, or a number of overstruck symbols.
2. Description of the Prior Art and Related Application
Control units of text stations in conventional text-printing devices are customarily provided with a microprocessor. A transfer of data words via a data bus consisting of several parallel lines generally takes place between one or more input units, the control unit, and one or more output units. Such devices customarily employ a keyboard, a receiver or a memory as an input unit and a printing unit, a transmitter, and/or a further memory unit as an output unit. By means of the printing unit, characters are represented as symbols on a recording medium, generally on paper. The printing unit may contain a printing means such as a hard type printing mechanism or a mosaic printing mechanism. The characters to be represented by means of the printing unit are transferred within the text station via the data bus and are encoded by means of one or more data words.
If the printing unit is designed as a hard type printing mechanism, each type character is called by means of a specific code word. If the type printing mechanism employs, for example, a type wheel which has a plurality of spokes with a specific type symbol at the respective ends thereof, the code words correspond to the spoke numbers. If the printing means is a mosaic printing mechanism, and the characters are stored in a character generator, the code words correspond to the addresses of the characters in the character generator.
Generally the data words are different from the code words so that a conversion from the data words to the code words is necessary. For this purpose a memory is provided in the text station at whose address inputs the data words are stored and at whose outputs the code words are released. A memory of this type can always be used when an individual symbol such as a hard type symbol or a symbol in the character generator is associated with each data word in a one to one relationship. In order to maintain the symbol sets as small as possible, however, printed characters are frequently comprised of a number of symbols which means that the character which is represented on the input means, such as a keyboard, will actually be comprised of more than one data word, each data word corresponding to the respective individual symbols which comprise the printed character. An example is the character "#", which can be comprised of two characters "/" overstruck or overprinted on the character "=". It is also possible to provide characters which are represented by means of several data words which are printed in sequence. In these two situations, it is not possible to automatically convert the data words into the corresponding code words for representing the printed symbol.
It is known from German AS No. 29 37 725, corresponding to co-pending U.S. application Ser. No. 172,074 filed on July 24, 1980 now U.S. Pat. No. 4,319,077 and assigned to the assignee of the present application, to convert the data words of those characters for which no code word exists into code words for symbols which describe the character which is encoded by means of the data word. For example, the data word for the character ".sctn." (or " " in English convention) can be converted into the code words for the symbols "par". Similarly, the data word for the character "$" can be converted into the code words for the symbols "dol". This conversion is undertaken in that when it is recognized that the character to be represented does not have a one to one relation with a printable symbol in a first memory, an address word stored in the first memory identifies an address location in a second memory in which the code words are stored for the described characters. This method and apparatus, however, represents only those characters which can be represented by a number of individual serially-printed symbols printed at several symbol locations which describe the corresponding character, and does not provide a means for representing characters which are to be represented by a number of overstruck or overprinted symbols.